Fungi of the genus Fusarium are responsible for numerous crop plant diseases around the world. These fungi are particularly harmful to cereal crops, such as corn, sorghum and wheat. For example, one or more species of Fusarium can be isolated in every season from nearly every vegetative or reproductive part of corn in every corn-growing region of the United States. A particular species of Fusarium can occur singly or as part of a complex of Fusarium species or fungi of other genera. Depending upon weather, soil and the particular cereal variety, damage to the crop can range from negligible to extensive. A common occurrence, for example, is for a Fusarium infected cereal crop to be damaged if it comes under stress. The diseases caused by Fusarium fungi include seedling blight, root rot, stalk rot, and ear rot.
Fusarium moniliforme is the most widely prevalent and economically important Fusarium species found on corn in the United States, and is the most frequently isolated Fusarium species in shelled corn. Seed lots with 100% infection are not uncommon. It is also the most frequent cause of ear rot, and has been implicated in seedling blight and root rot. One of the primary diseases affecting the corn crop in the United States is stalk rot, and F. moniliforme is considered to be the predominant stalk rot fungus in at least 10 states: Florida, Idaho, Iowa, Minnesota, Nebraska, New Jersey, North Carolina, Pennsylvania, South Carolina, and Virginia. F. moniliforme is also known to produce a phytotoxin.
Fusarium roseum is found less often in corn kernels than F. moniliforme, primarily occurring in the humid sections of the corn belt eas of the Mississippi River and along the Atlantic seaboard. F. roseum is a cause of ear rot and seedling blight, and one of the important causes of stalk rot in the New England, Mid-Atlantic and Northern corn belt states. F. roseum also produces mycotoxins in kernels and stalks.
Additional species of Fusarium fungi which are of lesser importance than F. moniliforme and F. roseum in corn disease include F. tricinctum, F. oxysporum, and F. soloni. While these species are mainly of importance as parts of disease complexes, F. tricinctum has been reported to produce mycotoxins in kernels and stalks.
Fusarium fungi are not believed to be strong pathogens. Generally, they produce major symptoms and damage crops only when the crops are under stress. The advance of the fungi in the plant is determined largely by what stress is occurring and the rate of cell death in plant tissues. Fusarium fungi has a wide host range of cereals and grasses and very effective methods of survival in soil or plant residues. Wherever grasses or cereals are grown in the United States, a Fusarium infection of seedlings, roots, stalks, or ears is always possible. Thus, a method of controlling Fusarium infection in cereal crops is highly desirable. See generally, Fusarium: Diseases, Biology and Taxonomy (P. E. Nelson, T. A. Toussoun, and R. J. Cook eds. 1981); Christensen and Wilcoxson (1966) Amer. Phytopathol. Soc. Monogr. No. 3, 59 p.; Koehler (1959) Illinois Agric. Exp. Stn. Bull. 639. 87 p.; Koehler (1960) Illinois Agric. Exp. Stn. Bull. 658. 90 p.
Other plant pathogenic fungi which have significant impact include Pythium spp., Phytophthora spp., Sclerotinia spp., Sclerotium spp., Rhizoctonia spp., and Colletotrichum spp., Pythium spp. are the causive agents of a wide range of diseases on many crop plants, including damping off, hollow stem, stem rots, root rots and wilts. Important strains include, among others, P. ultimun, P. debaryanum and P. aphanidermatum. Phytophthora spp. are also associated with a variety of plant diseases including damping off, leaf blight, petiole and stem infection and root infections. Plant pathogenic strains include, among others, Phytophthora porri, P. capsici, P. cryptogea, P. mexicana, P. cinnamomi (root decay) and P. infestans (late blight in tomatoes). Sclerotinia spp., causative agents of white rot, are widely dispersed and have a wide host range. Important plant pathogens include Sclerotinia sclerotiorum, S. minor and S. intermedia. Sclerotium spp. and S. rolfsii in particular are associated with stem root rots and blights (Southern blight). These pathogens affect many crops including rice, peanut and sunflower. Rhizoctonia spp. (literally root killers) are associated with root rots. Rhizoctonia are the mycelia stage of Basidiomycetes. The basidial stage is often designated by another name. Rhizoctonia solani (basidial stage: Thanatephoris cucumeris) is the most important member of this group which in particular affects cereals (wheat and oats), sunflower and cotton. Rhizoctonia violacea (basidial stage: Helicobasidium purpureum) is the causative agent of violet root rot. Colletotrichum spp. are associated with anthracnose disease in a number of plants. Colletotrichum graminicola affects cereals and grasses; C. coccodes affects vegetables including tomatoes and potatoes; C. lagenarium affects cucumbers and melons, in particular, and C. lindemuthianum affects Phaseolus bean.
Attempts have been made to control fungal infections of plants by biological means. For example, it is reported in Kommedahl and Mew, (1975) Phytopathology 65: 296-300, that three corn hybrids were coated with Bacillus subtilis and Chaetomium globosum to determine the effect on seedlings, final stands, stalk rot, and yields in the field, in comparison with the standard chemical seed treatment, captan. Stalk rot and breakage were less with the organism- and captan-coated seeds compared to non-coated seeds. Grain yields, however, were generally higher for the captan-coated seeds than for the organism-coated seeds. It was concluded that the organisms were not as consistent as captan in protecting the plants, and that additional research was required before the use of organisms for coating corn kernels would be commercially feasible. For additional discussion of biological control of fungal infections in plants see generally: R. J. Cook and K. F. Baker, The Nature and Practice of Plant Pathogens (Amer. Phytopathol. Soc. 1983); K. F. Baker and R. J. Cook, BIOLOGICAL CONTROL OF PLANT PATHOGENS (Amer. Phytopathol. Soc. 1982); Burges, H. D. (ed.) (1981) Microbial Control of Pests and Plant Disease 1970-1980, Academic Press, New York; Baker, (1968) Ann. Rev. Phytopathol. 6: 263-294; Chang and Kommedahl, (1968) Phytopathology 58: 1395-1401; Kommedahl and Chang, (1966) Phytopathology 56: 885; Kommedahl et al., (1974) Ann. Proc. Am. Phytopathol. Soc. 1: 46; Mew and Kommedahl, (1972) Plant Dis. Rep. 56: 861-863; Mitchell, (1973) Soil Biol. Biochem. 5: 721-728; Papavizas, (1973) Soil Biol. Biochem. 5: 709-720.
In Kawamoto and Lorbeer, (1976) Plant Dis. Reptr. 60: 189-191, it is reported that onion seedlings were protected from damping-off, caused by a particular strain of Fusarium oxysporum, by infesting the onion seedlings with Pseudomonas cepacia Burkh strain 64-22. This P. cepacia strain (64-22) was reported to be recovered from the root, root-stem zone and seed coat on 18-day old seedlings from inoculated seeds. Live cells of P. cepacia 64-22 were reported to inhibit Fusarium oxysporum f. sp. cepae, while dead cells and culture filtrates did not. The authors stated that the mechanism through which P. cepacia protects young seedlings was open to speculation. The authors concluded that the experiments reported at least supported the feasibility of biological control measures to improve onion seedling stand, but that "at present we could not recommend infesting onion seed with P. cepacia for commercial plantings . . . ", presumably because some strains of P. cepacia have been reputed to be pathogenic to onions.
R. D. Lumsden reported in an abstract in Phytopathology 72: 709 (1982) that a strain of P. cepacia is antagonistic to Pythium aphanidermatum and protects cucumber seedlings from infection by this fungus in soil. In U.S. patent application Ser. No. 500,043, filed June 1, 1983, now U.S. Pat. No. 4,588,584, by R. D. Lumsden and Myron Sasser, they describe the protection of cucumber and peas from Pythium disease by use of a new biotype of P. cepacia designated SDL-POP-S-1. Protection is afforded through bacterial inoculation of seeds.
Another strain of Pseudomonas cepacia protected China Aster against wilt caused by Fusarium oxysporum f. sp. callistephi in greenhouse and field tests (T. D. Cavileer and J. L. Peterson, Abstract No. 522, American Phytophathological Society Annual Meeting, 1985).
Kommedahl and Mew, supra, also reported that captan is widely used as a seed treatment because it is reliable, easy to apply, and inexpensive. It was also reported, however, that captan is not always a good seed treatment under all conditions. In particular, Kommedahl and Mew, supra, reported that under prolonged conditions of low soil temperature and high soil moisture, biological controls proved superior to captan in reducing root infections. This was attributed to the possible multiplication of organisms and their growth from the seed to the root surface. Thus, despite the general effectiveness of captan and the lack of commercial prospects for biological control reported in Kommedahl and Mew, supra, it would be highly desirable to develop a method of biologically controlling fungal infections in cereal crops, particularly corn.